The present invention relates to a packaging film, and more particularly to a laminate useful as a lidstock for sealing a tray closed.
It is common in food packaging operations for a food product, such as fresh meat, to be placed on a tray, such as a thermoformed expanded polystyrene tray having a central depressed area and a surrounding peripheral flange. A thermoplastic film or laminate may then be positioned over the food and heat sealed to the peripheral flange to hermetically enclose the food product. In such arrangement, the thermoplastic film or laminate is the xe2x80x9clidxe2x80x9d or xe2x80x9clidstockxe2x80x9d and the tray is a xe2x80x9csupport member.xe2x80x9d
The lidstock should be capable of forming a strong, hermetic seal with the support member. This is true even where the sealing area of the tray may be exposed or contaminated with by-product (e.g., meat purge) from the packaged food. This is also true where, as is commonly the case, the support member is relatively rigid. Heat sealing a flexible lidstock to a rigid support member is more challenging than heat sealing the flexible lidstock to either another flexible film or laminate or to itself (for example, in a fin seal arrangement commonly used in vertical form-fill-seal operations).
To heat seal the lid to the support member, a heated bar engages the outside of the lid to compress it against the flange of the support member. In so doing, heat transfers from the heated bar to the outside of the lid, through the thickness of the lid, to the inside sealant layer of the lid, and to the flange of the support member. The resulting heat and compression causes the contacting surfaces of the lid and support member to become molten and to intermix with one another. The heating bar is then removed to allow the sealed area to cool and form a sealed bond.
The heat from the heat seal process may also cause a heat-shrinkable lidstock to shrink or create a shrink tension in the areas of the lidstock that have been exposed to a sufficient amount of heat for a sufficient amount of time to effect a shrink.
The seal strength of the resulting sealed package may be determined by several methods. The support member may be pierced with an inflation needle and the interior of the sealed package may then be inflated until the lid or seal between the lid and support member fails. A higher internal inflation pressure at failure indicates a stronger seal strength. Alternatively, the sealed package may be placed in a vacuum chamber and subjected to decreasing external pressures until failurexe2x80x94a lower external pressure at failure indicating a stronger seal strength. Also, a representative sample of the seal may be cut from the sealed package (or formed separately) so that the lidstock may be pulled from the support member, for example, using an Instron tensile tester under specified conditions. A higher maximum force attained before failure indicates a stronger seal strength.
In all of these tests of seal strength, the failure mechanism may occur in one or more of several ways. In each case, the failure mode seeks a failure path requiring the least amount of force. For example, the bond between the lidstock and the support member may fail adhesively so that the lidstock simply peels away from the support member. Or, the lidstock may fail cohesively along a path cutting generally perpendicularly through one or more layers of the lidstockxe2x80x94and then fail adhesively along the interface between two layers of the lidstock. The failure path may combine an intricate path of cohesive and adhesive failuresxe2x80x94all while the lidstock is being stretched by the applied forcexe2x80x94to present a complicated failure mode.
The above discussion is made to establish that a weaker cohesive strength within a layer of the lidstock and/or a weaker adhesive bond strength between layers of the lidstock may weaken the seal strength of the sealed package. This is especially true where the seal strength failure mode is not simply the peeling of the lidstock from the support member by adhesive failure of the sealing bond between the lid and the support member.
A desirable lidstock provides gas (e.g., oxygen, carbon dioxide) barrier attributes sufficient to enhance the storage life of the packaged food. The barrier characteristics of the lidstock may have increased importance where the interior atmosphere of the package may be modified, for example, to decrease the concentration of oxygen from that of ambient air or to increase the concentration of oxygen and carbon dioxide from that of ambient air. For example, in packaging meat, the atmosphere in the sealed package may comprise about 80% by volume oxygen and about 20% by volume carbon dioxide in order to inhibit the growth of harmful microorganisms and extend the time period in which the meat retains its attractive red (xe2x80x9cbloomxe2x80x9d) coloration. Oxygen and carbon dioxide barrier attributes may be imparted to a film by incorporating, for example as a film layer, one or more resins having low permeability to oxygen. (Since carbon dioxide barrier properties generally correlate with oxygen barrier properties, only oxygen barrier properties are discussed in detail herein.)
It is not unusual for the inter-layer bond strengths associated with the incorporation of barrier resins or barrier layers into a lidstock to be weaker than the inter-layer bond strengths that would be present if the barrier resin or layer were absent. That is to say, the inter-layer bond strength between a barrier layer and an adjacent layer is usually the weakest inter-layer bond strength of a film. It is also possible that weaker inter-layer bond strengths may be associated with one or more xe2x80x9ctie layersxe2x80x9d that may accompany the use of a barrier layer. Although a tie layer may be inserted between the barrier layer and an otherwise adjacent film layer in order to improve the inter-layer bond adhesion, the resulting bond strength between the tie layer and its adjacent film layer may be less than the bond strength between the tie layer and its adjacent barrier layer. Accordingly, the tie layer may present the weakest inter-layer bond strength of the lidstockxe2x80x94and thus present the failure path during a seal strength test.
In order to produce packaged product at a fast (and therefore economical) rate, the lidstock should be capable of being quickly heat sealed to the support member. A lidstock that facilitates quick heat sealing is said to have good xe2x80x9csealability.xe2x80x9d
It is also desirable for the lidstock to be printed. Such printing provides important information to the end-user of the packaged foodxe2x80x94information such as the ingredients of the packaged food, the nutritional content, package opening instructions, food handling and preparation instructions, and food storage instructions. The printing may also provide a pleasing image and/or trademark or other advertising information to enhance the retail sale of the packaged product.
Such printed information may be placed on the outside surface of the lidstock. However, such surface printing is directly exposed to a heated bar during the heat seal operation that seals the lid to the support member. As a result, the surface printing may become smeared or otherwise degraded. A surface printing is also exposed to other physical abuses during distribution and display of the packaged product. Such abuse may also degrade the clarity and presentation of the printed image.
Once the lidstock has been sealed to a support member to form a closed package, it is also desirable that the lidstock not appear wrinkled or wavy. Such wrinkles or waves tend to form in the corner areas of a sealed lidstockxe2x80x94and may appear even if the sealed lidstock forms a tight, xe2x80x9cdrum-likexe2x80x9d lid for the support member. The wrinkles may also appear as relatively small film corrugations near the sealed area of the lid, in particular at the leading and trailing ends (relative to the machine direction) of the package. Such wrinkles or waves cause the package to present a less than desirable appearance to the customer.
The present invention addresses one or more of the aforementioned problems. A laminate is provided that comprises a first film having an inside surface and an outside surface opposite the inside surface of the first film. The first film comprises a sealant layer forming the inside surface of the first film. The sealant layer comprises one or more polymers each having a given melting point. At least one polymer of the sealant layer has the lowest melting point of the one or more polymers in the sealant layer. The laminate also comprises a second film having an inside surface and an outside surface opposite the inside surface. The second film comprises an outside layer forming the outside surface of the second film. The outside layer comprises at least about 40% by weight of the outside layer of one or more relatively high-melt polymers each having a melting point at least about 25xc2x0 F. higher than the lowest melting point polymer of the sealant layer. The laminate also includes a printed image between the first and second films. The first film has an oxygen transmission rate greater than the oxygen transmission rate of the second film, measured (at standard temperature and pressure) per square meter per day per 1 atmosphere of oxygen pressure differential measured at 0% relative humidity and 23xc2x0 C. The laminate has an oxygen transmission rate of no more than about 100 cubic centimeters (at standard temperature and pressure) per square meter per day per 1 atmosphere of oxygen pressure differential measured at 0% relative humidity and 23xc2x0 C. The laminate has a free shrink in each of the transverse and machine directions of at least about 10% at 200xc2x0 F. and at least about 21% at 240xc2x0 F. The laminate is heat sealed to a support member to form a closed package.
Heat sealing a laminate to a support member (e.g., tray) where the laminate has the recited free shrinks at both 200xc2x0 F. and 240xc2x0 F. reduces the amount and size of wrinkles (e.g., lid corrugations near the sealing area) and/or waves that may otherwise form in the lid of the resulting sealed package.
The laminate may provide enhanced seal strength relative to laminates having a barrier layer as an inner layer of the sealant film of the laminate. It is believed that a barrier layer often presents a weaker inter-layer bond strength relative to the inter-layer bond strengths of the other layers. When a packaging seal fails, it is typically because of a delamination between layers having the weakest inter-layer bond strength within a film of the laminate.
By placing the barrier layer in the outside film of the laminate, the relatively weaker inter-layer bond strength may be positioned farther from the bond between the laminate and the support member. When such potential inter-layer failure is farther from the inside (i.e., food-side) of the laminate, the failure tear propagation (i.e., the path of inter-film cohesive failure) must travel farther to reach the xe2x80x9cweakest linkxe2x80x9d inter-layer delamination path. This farther distance in the present invention is believed to enhance the seal strength.
Further, the placement of barrier components in the outside film of the laminate may allow for greater flexibility in manufacturing the lidstock laminate. This is because the inside sealant film may be manufactured without the additional restriction associated with accommodating barrier components in a coextruded, oriented film. For example, the extrusion of a barrier layer often requires higher temperatures than those needed to extrude the other layers of the film. This higher temperature associated with a barrier layer may limit the amount of lower melting point materials that can be used in the filmxe2x80x94otherwise, the film may flow too easily and the melt strength may be lowered to an unacceptable level for processing. Also, the orientation of a film having a barrier layer may require a higher orientation temperature, which can soften lower melting point materials in the film to an unacceptable level, causing an unstable orientation or welding together of adjacent layers. Thus, the incorporation of the barrier components in the outside film allows greater choices in imparting the desired shrink and other attributes to the inside, sealant film of the laminate.
The laminate may incorporate a trap print arrangement, which enhances the protection of the printed image of the laminate during the heat seal process that seals the laminate to a support member.
The laminate provides a low rate of oxygen transmission, which enables the atmosphere within the sealed package to be modified to extend the shelf life and bloom xe2x80x9ccolor lifexe2x80x9d of a packaged red meat product. The laminate also provides excellent print quality and optical clarity.
The laminate can provide excellent sealability to a support member. This allows a packager to run the heat sealing machine at a fast rate while also providing good seal strength between the laminate and the tray. The resulting seal between the laminate and the tray may provide excellent strength even where the seal is formed in the presence of contaminants and under variable heat sealing temperatures.
These and other objects, advantages, and features of the invention will be more readily understood and appreciated by reference to the detailed description of the invention and the drawings.